1. Field of the Invention
The present invention relates to a light source apparatus emitting white light, and a liquid crystal display using such a light source apparatus.
2. Description of the Related Art
In current television receivers (TVs), NTSC (National Television System Committee) system color displays are standard. When the color reproduction range of a display is narrower than that of NTSC, an image with a narrower color range than the colors of original signals is displayed, thereby the dynamic range is impaired to cause degradation in image quality. Recently, in a liquid crystal display (LCD) which is a kind of display, the expansion of the color reproduction range has been attempted.
As an indicator of the color reproduction range, a so-called NTSC ratio is used. The NTSC ratio is a ratio of the area of a color range which can be reproduced by a display with respect to the area of the reproduction range of NTSC in an xy chromaticity diagram, and the NTSC ratio is represented by the following formula (1):NTSC ratio (%)=(area S of triangle of display)/(area SN of NTSC triangle)  (1)
A specific method of calculating the NTSC ratio will be described below.
The area S of a triangle representing the color reproduction range in a chromaticity diagram is determined by the following formulas (2) through (6):Side RG=[(XR−XG)2+(YR−YG)2]1/2  (2)Side GB=[(XG−XB)2+(YG−YB)2]1/2  (3)Side BR=[(XB−XR)2+(YB−YR)2]1/2  (4)Half-perimeter P=(side RG+side GB+side BR)/2   (5)Area S=[half-perimeter P]1/2  (6)
In this case, (XR, YR), (XG, YG) and (XB, YB) represent the vertex coordinates of red (R), green (G) and blue (B) (R chromaticity point, G chromaticity point and B chromaticity point), respectively in the triangle in the chromaticity diagram. The sides RG, GB and BR represent a distance between the R chromaticity point and the G chromaticity point, a distance between the G chromaticity point and the B chromaticity point and a distance between the B chromaticity point and the R chromaticity point, respectively. In the NTSC system, each vertex coordinate is as follows, so the area SN is SN≈0.157.XR=0.67, YR=0.33XG=0.21, YG=0.71XB=0.14, YB=0.08
On the other hand, the vertex coordinates (chromaticity points) of a triangle representing the color reproduction range of a target liquid crystal display panel is determined from spectrum data as follows. Assuming that CIE1931 XYZ color matching functions are x(λ), y(λ) and z(λ), and the spectrum of a backlight source is, P(λ), and the spectral transmittance of each color filter of the LCD panel is T(λ), tristimulus values (X, Y, Z) are determined by the following formulas (7) through (9). In the formulas, A represents the wavelength of light.X=∫p(λ)x(λ)T(λ)dλ  (7)Y=∫p(λ)y(λ)T(λ)dλ  (8)Z=∫p(λ)z(λ)T(λ)dλ  (9)
The ratios x, y and z of an X component, a Y component and a Z component are determined from these tristimulus values.x=X/(X+Y+Z)y=Y/(X+Y+Z)
Then, (x, y) of each color is plotted as each chromaticity point (XR, YR), (XG, YG) or (XB, YB), thereby a triangle representing the color reproduction range of the liquid crystal display panel can be obtained. The area S of the triangle is determined by the above-described formulas (2) through (6), and the NTSC ratio is determined by the formula (1).
In a transmissive illumination system liquid crystal display, as a backlight source, a cold cathode fluorescent tube (CCFL-tube) emitting white light has been heavily used hitherto. In the CCFL tube of this kind, in general, an R phosphor, a G phosphor and a B phosphor which emit R light, G light and B light, respectively are applied to an inner tube wall surface, and ultraviolet radiation generated by exciting a sealed noble gas or the like excites each phosphor to emit each color light.
However, in a liquid crystal display using a CCFL tube in related arts as a light source, there is an issue that mainly due to its light source emission spectrum, the NTSC ratio is low, thereby the color range is narrow, or an issue in color balance. More specifically, the color B does not have an issue because the color B exceeds the range of the NTSC system; however, in the color G and the color R, the chromaticity ranges of the colors G and R tend to be narrow, so an improvement in display is desired.
In recent years, to improve the color reproduction by expanding a color range or improving color balance, a technique of combining a CCFL tube and another light source has been proposed (refer to Japanese Unexamined Patent Application Publication Nos. 2004-139876, 200544756, 2004-294984 and 2004-163902).